1. Field of the Invention
This invention relates to a pressure sensor utilizing the magnetostriction effect of an amorphous magnetic alloy.
2. Description of the Prior Art
The inventors have earlier proposed a pressure sensor utilizing the magnetostriction effect of an amorphous magnetic alloy (U.S. Pat. No. 4,938,069).
FIG. 7 is a schematic vertical cross-sectional view of one example of such pressure sensor. This pressure sensor comprises a cylindrical base or body 1 made of titanium, a film 7 of an amorphous magnetic alloy mounted around the outer periphery of the body 1, a cylindrical tubular bobbin 10 disposed radially outwardly of the film 7 and mounted around the outer periphery of the body 1, a pressure detector coil 8 wound on the outer periphery of the bobbin 10, a dummy coil 9 wound on the outer periphery of the bobbin 10, a cup-shaped yoke 11 mounted radially outwardly of the two coils 8 and 9, and a detector unit 13 electrically connected to the two coils 8 and 9. The body 1 has two central holes separated from each other by a partition wall 1a, the two central holes defining a pressure chamber 3 and a dummy chamber (or reference chamber) 6, respectively. The body 1 also has a pressure introducing opening 2 defined by an open end of the pressure chamber 3, a greater-diameter flange 1b formed on the outer periphery of the body 1 and disposed adjacent to the end of the body 1 having pressure introducing opening 2, a recess (smallest-diameter portion) 14 formed in the outer periphery of the body 1 and extending throughout the entire length of the smaller-diameter portion of the body 1 between the end portion (the upper end portion in FIG. 7) of the body 1 remote from the pressure introducing opening 2 and the greater-diameter flange 1b, and a fixing screw thread 12 formed on the outer periphery of the end portion (the lower end portion in FIG. 7) of the body 1 close to the pressure introducing opening 2. A wall part which defines part of the pressure chamber 3 and exists between the pressure chamber 3 and the recess 14 is a deforming part 4 which is deformed in response to a change in the pressure within the pressure chamber 3. Another wall part, being integrated with the above-mentioned wall, which defines part of the dummy chamber 6 and exists between the dummy chamber 6 and the recess 14 is a non-deforming part 5 which is not influenced by the pressure in the chamber 3. The film 7 which is a rectangular thin sheet of an amorphous magnetic alloy is disposed in the recess 14, and is wound around the outer periphery of the body 1, and is fixedly bonded thereto by an adhesive under pressure. The depth of the recess 14, that is, the height of steps 14a and 14b, is determined to be equal to the sum of the thickness of the wound film 7 and the thickness of the adhesive layer. The fixed bonding of the film 7 by application of pressure is carried out by fitting a tube 21 of a heat-shrinkable resin on the film 7 and then by heating this tube (see FIGS. 8a to 8c). After this bonding operation, the tube is removed. The steps 14a and 14b serve as positioning means which prevents the film 7 from being displaced along the axis of the body 1 during the fixed bonding of the film 7 by the adhesive. The bobbin 10 having two peripheral grooves in its outer peripheral surface covers the recess 14, and is fitted on the body 1. The pressure detector coil 8 is received in one of the two peripheral grooves close to the flange 1b, and the dummy coil 9 is received in the other peripheral groove. The two coils 8 and 9 are used as permeability detector elements, and cooperate with the amorphous magnetic alloy film 7 to form a magnetic circuit.
The pressure of a fluid to be measured is fed to the pressure chamber 3 via the pressure introducing opening 2, and applies a force in a direction to expand the deforming part 4 defining the pressure chamber 3. As a result, the part 4 is deformed, so that the permeability of the amorphous magnetic alloy film 7 adhesively bonded to the outer surface of the deforming part 4 is changed. This change of permeability is detected by the pressure detector coil 8 as a change in inductance, and the pressure change is obtained by a differential output between the coil 8 and the dummy coil 9.
In manufacturing this pressure sensor, the yield rate (the ratio of products coming up to the standard, of which zero-point drift and sensitivity change due to change of temperature are less than .+-.0.1% Full Scale/.degree.C. respectively) of the pressure sensor has been as low as about 30%. This is due to the uneven thickness of the adhesive layer between the amorphous magnetic alloy film 7 and the body 1 and also to the bonding step. Namely, in the above construction, when the layer of the adhesive applied between the body 1 and the amorphous magnetic alloy film 7 is uneven from one place to another, the excess adhesive can not escape, and the resulting layer of the cured adhesive is irregular in thickness. For bonding the amorphous magnetic alloy film 7 to the body 1, the tube 21 of a heat-shrinkable resin is used. However, the degree of heat-shrinkage of this type is inevitably irregular. This condition is shown in FIGS. 8a to 8c which are enlarged views of that portion of FIG. 7 indicated by a dotted-line. The heat-shrinkable tube 21 is not always shrunk uniformly, and is distorted right or left, or bulges at its central portion. Therefore, the thickness of the adhesive layer 22 inside the tube 21 becomes irregular, which lowers the yield rate in manufacturing the pressure sensor.
When the pressure sensor of the above construction was dipped in hot water at 80.degree. C., rust was generated on the surface of the amorphous magnetic alloy film 7 in about one day, thus exhibiting a poor corrosion resistance. Further, since the amorphous magnetic alloy film 7 has a shape as shown in FIGS. 9 and 10, a gap 23 (where the amorphous magnetic alloy film 7 is absent, and hence only the adhesive layer is present) is formed after this film 7 is adhesively bonded to the body 1. This pressure sensor was subjected to a heat cycle test (cycle of -40.degree. C. and 150.degree. C.), and as a result, tongue-shaped exfoliation parts 24 of the adhesive layer developed respectively at the opposite ends of the gap 23 after the heat cycle test was conducted 500 times. Thus, the durability of the pressure sensor was lowered.
As described above, the pressure sensor of the above construction has problems such as a low yield rate and poor corrosion resistance and durability.